After Elder's HP runs out, Mind Parasite appears immediately. He often casts strong elemental spells like Firaga, Blizzaga and Thundaga. They deal about 900 damage to a front-row member if cast on all characters and near 1800 if cast on a single target. Shellshock makes those spells more effective and damages the characters even more. Forsaken drains MP and can inflict Confuse at the same time while Mind Blast inflicts Sap and Paralyze. His most powerful attack to the party is Soul Purge.Defense Down status.
The party may need to be healed immediately after Mind Parasite appears since the fight between Elder and this boss happen consecutively. Forsaken, Mind Blast and Shellshock can be troublesome so make sure Basuna, Shell and Chakra are ready. Also, his elemental spells can be harmful so Curaja and Cure Waltz are essentials. When he begins casting Soul Purge, Mind Parasite has to be finished off quickly to prevent more damage.
Parasitism is a non-mutual symbiotic relationship between species, where one species, the parasite, benefits at the expense of the other, the host.